This invention relates generally to control systems, and more specifically to a system and method of controlling shape memory alloy actuators.
Shape memory alloy (SMA) actuators have the potential for use in a wide range of actuator applications including space stations, automotive components, surgical devices and robotic arms. Such SMA actuators are described more fully in U.S. Pat. No. 6,318,070 B1 issued on Nov. 20, 2002 and entitled xe2x80x9cVariable Area Nozzle For Gas Turbine Engines Driven By Shape Memory Alloy Actuatorsxe2x80x9d; U.S. patent application Ser. No. 09/824,419 filed Apr. 2, 2001 and entitled xe2x80x9cVariable Area Nozzle For Gas Turbine Engines Driven By Shape Memory Alloy Actuatorsxe2x80x9d; and U.S. patent application Ser. No. 09/517,938 filed Mar. 3, 2000 and entitled xe2x80x9cShape Memory Alloy Bundles And Actuatorsxe2x80x9d, the disclosures of which are herein incorporated by reference.
Resistive heating is used as the means to induce martensite to austenite transformation in the material, which produces recovery stress. This stress can be used for actuation. By intelligently controlling the power input to the SMA actuator, the required temperature and hence the required amount of stress can be achieved. Since the power requirements may vary subject to such unknowns as ambient temperature, hysteresis and material degradation, open loop control (i.e., supplying predetermined power for a specific desired position) does not work; one has to resort to closed loop feedback control.
A closed loop feedback control system is required to achieve this objective. The challenge in devising an effective control algorithm for such a position control system is the nonlinear hysteretic behavior of SMA, which renders conventional linear control laws such as proportional integral derivative (PID) less effective. Conventional (PID type) SMA controllers have been applied to thin SMA wires. Because of the small size of the SMA wires, and therefore the consequent fast heating/ cooling behavior of the wires, PID can be employed with reasonable success. However, a large bundled cable actuator such as, for example, an actuator having 266 wires of 0.02xe2x80x3 diameter each, cannot be controlled by conventional PID type feedback control.
Another drawback is the bandwidth limitation of SMA actuators usually as the size (mass) increases, the dynamic response of the SMA becomes poorer. The high thermal energy input necessary to heat up the SMA to realize the necessary strain impedes fast actuation. In situations where active cooling is not an option for the control scheme, the hysteresis of the material makes it extremely difficult to quickly correct errors because of temperature overshoot. Once the SMA contracts more than necessary (too much power is supplied), it takes a long time to cool off sufficiently so that its length assumes the desired value. This imposes further restriction on the control algorithm for minimizing the overshoot error.
A powerful control approach is to employ a mathematical model of the hysteresis behavior of shape memory material to determine the control action. Since material property is subject to change over time, such models need to be updated continuously through real-time measurements and adaptive algorithms. A drawback with such an approach is that it is complex and computationally intensive, and consequently time consuming and expensive to implement.
In view of the foregoing, it is a general object of the present invention to provide a method of controlling an SMA actuator which avoids the above-described drawbacks associated with prior art SMA actuator controllers.
In a first aspect of the present invention, a method of controlling a shape memory alloy (SMA) actuator comprises the steps of supplying maximum control voltage to an SMA actuator where an object having a position to be controlled by the SMA actuator were to move toward a target position upon supply of non-zero control voltage, and the instantaneous actual position of the object is at a distance above a predetermined threshold from the target position. A variably controlled voltage is supplied to the SMA actuator between the maximum voltage and about zero voltage where the object to be controlled were to move toward the target position upon supply of the variably controlled voltage, and the instantaneous actual position of the object is at a distance below the predetermined threshold from the target position.
In a second aspect of the present invention, a method of controlling an SMA actuator comprises the steps of providing an SMA actuator for moving an object to a predetermined target position. A maximum control voltage is supplied to the SMA actuator where an object having a position to be controlled by the SMA actuator were to move toward a target position upon supply of nonzero control voltage, and the instantaneous actual position of the object is at a distance above a predetermined threshold from the target position. A variably controlled voltage is supplied to the SMA actuator between the maximum voltage and about zero voltage where the object to be controlled were to move toward the target position upon supply of the variably controlled voltage, and the instantaneous actual position of the object is at a distance below the predetermined threshold from the target position.
In a third aspect of the present invention, an SMA actuator control system comprises an SMA actuator including SMA wires and at least one position sensor. The SMA actuator is to be coupled to an object having a position to be moved by the SMA actuator. The system includes means for supplying maximum control voltage to the SMA actuator where the object having a position to be controlled by the SMA actuator were to move toward a target position upon supply of a non-zero control voltage, and the instantaneous actual position of the object is at a distance above a predetermined threshold from the target position. The system further includes means for supplying a variably controlled voltage to the SMA actuator between the maximum voltage and about zero voltage where the object to be controlled were to move toward the target position upon supply of the variably controlled voltage, and the instantaneous actual position of the object is at a distance below the predetermined threshold from the target position.
An advantage of the present invention is that the power to the SMA actuator is cut off prior to the controlled object reaching the target position so as to prevent overshoot of the target position.
These and other advantages of the present invention will become more apparent in the light of the following detailed description and accompanying figures.
FIG. 1 is a flow diagram of a process of controlling an SMA actuator in accordance with the present invention.
FIG. 2 schematically illustrates an SMA actuation system where SMA bundled cable actuates a mechanical member by means of the process of FIG. 1.
FIG. 3 is a graph of the relationship of strain vs. temperature of typical SMA material to illustrate the material""hysteretic behavior.
FIG. 4 is a graph of the response of the mean flap position for a step input command for both a conventional PID and the process of FIG. 1.
FIG. 5 is a graph illustrating SMA controller performance by the process of FIG. 1 with two different sets of controller parameters.
FIG. 6 is a graph illustrating PID controller performance with two different sets of parameters.